German Patent Application No. 41 36 486 describes a ballast for starting and operating a.c. high-pressure gas-discharge lamps, the ballast is supplied from a d.c. voltage source, for example, from the battery of a motor vehicle. In this context, initially a stabilized and/or power-regulated d.c. voltage, having a higher voltage level, is produced by means of a d.c./d.c. converter from the d.c. voltage of a d.c. voltage source. Then the alternating current required to operate the lamp is produced from this voltage with the aid of a controlled bridge circuit. The bridge circuit can be operated as a direct-current and as an alternating-current circuit, and it forms the power-supply circuit for the high-pressure gas-discharge lamp. The bridge circuit is controlled by a control circuit, and an ignition device is provided to ignite the high-pressure gas-discharge lamp.
In this conventional ballast, in order to achieve a rapid and reliable ignition, as well as to rapidly and reliably achieve a stable burning state without the lamp flickering, during initial operation, the bridge circuit is operated as a d.c. circuit. A signal is generated by a status-defining device when the high-pressure gas-discharge lamp is successfully ignited. This signal is fed via a time-delay circuit to the control circuit for the bridge circuit, ensuring that, only after the requisite period of delay, the bridge circuit is switched over from d.c. operation to a.c. operation. The approach described above differs from the approach provided by the present invention.
For the open-loop control when starting a high-pressure gas-discharge lamp, the Vedilis curve is usually preset as the current-/voltage curve, in the sense of a reference curve. The Vedilis curve is described in the "System Specifications for Field Test" of the VEDILIS Eureka Project No. 273 on Page B 1/3, and it is preset as the current-/voltage characteristic for gas-discharge lamps used in motor vehicles. "Vedilis" in this connection is an acronym standing for "Vehicle Discharge Light System." Thereupon, for the closed-loop control of the lamp power of a gas-discharge lamp, the lamp voltage U.sub.L is measured in the start-up or burn phase and the corresponding lamp current I.sub.L, belonging to the respective lamp voltage U.sub.L, is determined from the Vedilis curve. This lamp current I.sub.L then serves as a reference value for the subsequent current closed-loop control circuit by means of which the lamp power is controlled in closed loop to a constant value such as 35 W.
To use a gas-discharge lamp in motor vehicle headlights it is usually necessary to provide as much illumination as possible within a short period of time after switching the control unit on. This is described as a rapid starting of the illumination. As shown by the above-described Vedilis curve, for this purpose, the gas-discharge lamp can be operated with a specific overload immediately after being turned on. In this context, this overload is driven back as a function of the lamp voltage. The lamp should not be destroyed or have its service life adversely affected as a result of such an overload. This requirement must be observed for all operating states. Furthermore, the control must be selected such that the light output does not swing back again from an already attained value, because this is generally perceived as an unpleasant flickering. This swinging back of the light output is also characterized as a light-saddle effect.
It has been shown in practice that a control in accordance with the above-mentioned Vedilis curve cannot avoid generating light-saddles. Consequently, alternative methods are sought. It is generally possible to conduct the overload operation of the gas-discharge lamp by using a simple timing. In this connection, however, it should be seen as particularly disadvantageous that the respective state of the gas-discharge lamp is fully ignored. This can result in the destruction of the gas-discharge lamp or, at least, in a shortening of its service life.